1. Field of the Invention
This invention relates generally to boring tools, and more particularly to apparatus for steering percussion boring tools.
2. Description of the Prior Art
Utility companies often find it necessary to install or replace piping beneath different types of surfaces such as streets, driveways, railroad tracks, etc. To reduce costs and public inconvenience by eliminating unnecessary excavation and restoration, utilities sometimes use underground boring tools to install the new or replacement pipes. Existing boring tools are suitable for boring short distances (up to 60 ft.), but are not sufficiently advanced to provide directional control for longer distances. This lack of control, coupled with the inability of these tools to detect and steer around obstacles, has limited their use to about 20% of all excavations, with the majority of the remaining excavations being performed by open-cut trenching methods.
Therefore, the development of an economic, guided, horizontal boring tool would be useful to the utility industry, since it would significantly increase the use of boring tools by removing the limitations of poor accuracy and by reducing the occurrence of damage to in-place utilities. Use of such a tool instead of open-cut methods, particularly in developed areas, should result in the savings of millions of dollars annually in repair, landscape restoration and road resurfacing costs.
Conventional pneumatic and hydraulic percussion moles are designed to pierce and compact compressible soils for the installation of underground utilities without the necessity of digging large launching and retrieval pits, open cutting of pavement or reclamation of large areas of land. An internal striker or hammer reciprocates under the action of compressed air or hydraulic fluid to deliver high energy blows to the inner face of the body. These blows propel the tool through the soil to form an earthen casing within the soil that remains open to allow laying of cable or conduit. From early 1970 to 1972, Bell Laboratories, in Chester, N.J., conducted research aimed at developing a method of steering and tracking moles. A 4-inch Schramm Pneumagopher was fitted with two steering fins and three mutually orthogonal coils which were used in conjunction with a surface antenna to track the position of the tool. One of these fins was fixed and inclined from the tool's longitudinal axis while the other fin was rotatable.
Two boring modes could be obtained with this system by changing the position of the rotatable fin relative to the fixed fin. These were (1) a roll mode in which the mole was caused to rotate about its longitudinal centerline as it advanced into the soil and (2) a steering mode in which the mole was directed to bore in a curved path.
The roll mode was used for both straight boring and as a means for selectively positioning the angular orientation of the fins for subsequent changes in the bore path. Rotation of the mole was induced by bringing the rotatable fin into an anti-parallel alignment with the fixed fin. This positioning results in the generation of a force couple which initiates and maintains rotation.
The steering mode was actuated by locating the rotatable fin parallel to the fixed fin. As the mole penetrates the soil, the outer surfaces of the oncoming fins are brought into contact with the soil and a "slipping wedge" mechanism created. This motion caused the mole to veer in the same direction as the fins point when viewed from the back of the tool.
Published information on the actual field performance of the prototype appears limited to a presentation by J. T. Sibilia of Bell Laboratories to the Edison Electric Institute in Cleveland, Ohio on Oct. 13, 1972. Sibilia reported that the system was capable of turning the mole at rates of 1.degree. to 1.5.degree. per foot of travel. However, the prototype was never commercialized.
Several percussion mole steering systems are revealed in the prior art. Coyne et al, U.S. Pat. No. 3,525,405 discloses a steering system which uses a beveled planar anvil that can be continuously rotated or rigidly locked into a given steering orientation through a clutch assembly. Chepurnoi et al, U.S. Pat. No. 3,952,813 discloses an off-axis or eccentric hammer steering system in which the striking position of the hammer is controlled by a transmission and motor assembly. Gagen et al, U.S. Pat. No. 3,794,128 discloses a steering system employing one fixed and one rotatable tail fin.
However, in spite of these and other prior art systems, the practical realization of a technically and cost-effective steering system has been elusive because the prior systems require complex parts and extensive modifications to existing boring tools, or their steering response has been far too slow to avoid obstacles or significantly change the direction of the boring within the borehole lengths typically used.